1. Field of the Invention
This invention relates to magnetostatic wave (MSW) filters and, in particular, the invention relates to an MSW filter in which a desired propagation mode is trapped for propagation in the MSW filter medium with a relatively low attenuation and undesired modes are sharply attenuated.
2. Description of Related Art:
In magnetostatic wave devices energy in the form of magnetic waves is carried through the bulk of a suitable MSW medium, for example, a ferrite material such as yttrium iron garnet (YIG). Typically, an input transducer such as a conductive microwave stripline carries an RF electromagnetic current or signal which, when magnetically coupled to the ferrite is converted to an MSW which propagates therein. The MSW may be reconverted to RF by means of an output transducer suitably coupled to the ferrite at a location along the axis of MSW propagation in the medium remote and isolated from the input transducer. Ferrite boundaries and metallization boundaries cause MSW reflections in the ferrite. When properly sized the boundaries form resonators and delay lines. In the delay line, for example, the MSW propagates from the input transducer to the output transducer. By appropriately biasing the medium with a magnetic field the boundaries, the transducer geometry and the field define a frequency selectivity in such a MSW delay line. Typically, because the dimensional boundaries of the delay line form a MSW waveguide capable of supporting many modes, input signals outside the desired passband of the lower order MSW mode can excite higher order modes which, if not attenuated, can travel to the output transducer and produce undesired responses within the filter stop band. Similarly, a resonator which may be a YIG medium having square boundaries causes the base frequency to resonate within the boundaries. However, higher order modes or harmonics of the wavelength of the base frequency also appear in the resonator output. While various techniques are known for varying the bandpass shape of the delay line or the resonator including carefully tailoring of the strip line and boundaries, harmonics or extraneous undesirable modes may be propagated in such devices thereby creating out-of-band responses which seriously degrade the filter performance.
Bandpass filters and filter banks based on MSW resonators and delay lines are attractive as preselectors in radar and communication systems and in channelized receivers. However, both the resonator and delay-line based filters require that undesired out-of-band responses due to higher order MSW modes be suppressed. This has been achieved in the delay line filter by the use of resistive aluminum strips, by tapering the width of the YIG film, or by the use of a lossy second layer of YIG which is coupled to the low loss YIG. In resonators, higher order modes have been suppressed by etching an annular ring in the YIG film. None of these techniques are sufficient to achieve an out-of-band rejection greater than about 60 dB, as required in high dynamic range systems. Moreover, the mode suppression approaches of delay line filters introduce undesirable excess loss.
The most troublesome responses are due to MSW width modes which are higher order MSW cavity type resonances and are defined by the edges of the YIG film.